1. Field of the Invention
The present invention relates to a spatial light modulator and a method for producing the same, and in particular, to a spatial light modulator suitable for use in a projection display apparatus, a holography television apparatus, an optical operation apparatus, and the like.
2. Description of the Related Art
In the field of high definition TV having pixels arranged at a high density for displaying an image on a large image plane, various constructions have been proposed and put into practical use. Projection display apparatuses using a liquid crystal display device instead of a conventional cathode ray tube (hereinafter, referred to as CRT) have actively been developed.
In a display apparatus using a CRT, a higher density of pixels results in a lower luminance of an image plane to darken an image. In contrast, a projection display apparatus using a liquid crystal display device driven by transistors has problems in that it is difficult to enhance the ratio of a total area of pixels with respect to an area of display and that the liquid crystal display device is expensive.
Today, a liquid crystal light valve using a CRT for optical input is a focus of attention for a simple construction and for having advantages of both of the CRT and the liquid crystal display device. An idea of the liquid crystal light valve is disclosed in Appl. phys. Lett. 17 (1970) p. 51 by Hughes Aircraft Company. This literature describes a liquid crystal light valve using ZnS as a photoconductive material and a twisted nematic liquid crystal. Another liquid crystal light valve using CdS as a photoconductive material and a twisted nematic liquid crystal is disclosed in Appl. Phys. Lett. 22 (1973) p. 90. Liquid crystal light valves using single crystalline silicon and a twisted nematic liquid crystal are disclosed in U. S. Pat. No. 4,913,531, Japanese Laid-Open Patent Publication No. 3-192332, and J. Appl. Phys. 57 (1985) p. 1356.
The use of a light valve including a highly sensitive light receiving layer (namely, photoconductive layer) formed of amorphous silicon and a liquid crystal material allows a moving image to be generated on a large image plane having a size of 100 in. (inches) or larger. Electro-optical devices including a light receiving layer formed of amorphous silicon and a twisted nematic liquid crystal are disclosed in Ashey et al., U.S. Pat. No. 4,693,561, Appl. Opt. 26 (1987) p. 241, and U.S. Pat. No. 4,538,884. Liquid crystal light valves using amorphous silicon and CdTe are disclosed in U. S. Pat. No. 4,799,773 and SID '90 17A. 2 p. 327 by Hughes Aircraft Company.
The use of a ferroelectric liquid crystal having a high response speed as a liquid crystal material realizes a liquid crystal light valve with a faster response and a higher resolution. Such a light valve utilizing the ferroelectric liquid crystal for an excellent memory function and a high bistability thereof is considered to have a critical role in optical computing, which is a future technology for parallel operation.
A spatial light modulator including a light receiving layer formed of amorphous silicon and having a diode structure and a ferroelectric liquid crystal is disclosed in Appl. Phys. Lett. 51 (1987) p. 1232 by a research group of Keio University as the first device of this kind. A construction for such a spatial light modulator is proposed in SPIE 754 (1987) p. 207 by the University of Colorado Foundation, Inc. Spatial light modulators developed by the University of Colorado Foundation, Inc. are disclosed in Appl. Phys. Lett. 55 (1989) p. 537 and U.S. Pat. No. 4,941,735.
A projection system for writing a TV image is disclosed in SID '91 13.3, p. 254 by Greyhawk Systems, Inc.
A holography television apparatus is also a focus of attention as an apparatus for displaying a three-dimensional image which can be seen without 3-D glasses. Especially, a liquid crystal display device is considered as an excellent rewritable hologram recording medium. For example, an electronic holography system was developed by Hashimoto et al. which uses a high density display device as a phase modulation type spatial light modulator. Such a system is disclosed in SPIE Proc. Vol. 1461, Practical Holography V (1991) pp. 291-302.
A liquid crystal display device driven by a transistor which is currently used has a resolution of 12 to 25 lp/mm. A spatial light modulator having a resolution of 200 1p/mm is now desired.
Liquid crystal light valves including a light receiving layer formed of single crystalline silicon and a liquid crystal material and having microscopic electrodes as pixels are disclosed in U.S. Pat. No. 4,913,531, Japanese Laid-Open Patent Publication No. 3-192332, and J. Appl. Phys. 5 (1985) p. 1356. According to this technology, the light receiving layer has a groove which is filled with a liquid crystal layer in order to reduce the level of crosstalk between adjacent pixels and improve the resolution. However, the light receiving layer is restricted to having a construction including a MOS (metal-oxide-semiconductor) structure or a Schottky junction. In the devices proposed in the above literatures, pixel portions are shielded from reading light by pixel electrodes, and an inter-pixel portion between the pixel portions are shielded from the reading light by a metal film.
A spatial light modulator including a single crystalline layer having a groove formed at a surface thereof and an electro-optical crystal layer in order to improve the resolution is disclosed in U.S. Pat. No. 4,619,501. When single crystalline silicon is used for the light receiving layer, the resolution has a limit since the standard thickness of the layer is 100 .mu.m and a corrugation having a height of several microns cannot be eliminated from the layer even by surface rubbing. Under these circumstances, a spatial light modulator having a light receiving layer formed of amorphous silicon is expected to realize a high resolution because such a light receiving layer can be formed on a glass substrate treated with highly precise optical rubbing and also can be formed in a thickness of several microns.
Examples of a liquid crystal light valve and a spatial light modulator both including a liquid crystal layer and a photoconductive layer, which are provided with microscopic electrodes and a light blocking film are disclosed in Japanese Laid-Open Patent Publication No. 62-40430 and Japanese Laid-Open Patent Publication No. 62-169120, respectively. The microscopic electrodes are advantageous in being easier to produce than a reflective layer formed of a multi-layer dielectric thin film, in having no dependency on incident angle, and in having a high reflective power. When such a conventional spatial light modulator is used in a projection display apparatus, a high definition TV image can be obtained because the spatial light modulator has a high ratio of a total area of pixels with respect to an area of display, and pixels of a clear shape.
Such a spatial light modulator is generally considered to need two types of light blocking films. The first one is an input light blocking film provided for prevent superimposition of the input light to be modulated on the output light. The second one is an output light blocking film provided for prevent reading light from being leaked to the light receiving layer and thus to prevent switching of the spatial light modulator. In the case when the light blocking films are provided between the light receiving layer and a reflective film, the light blocking films are formed of an insulating material. In the case when the light blocking films are directly provided on a transparent electrode, the light blocking films need not have electric insulation.
The followings are considered to limit the resolution of the spatial light modulator.
(1) Lateral diffusion of electrical carriers generated in the light receiving layer;
(2) Drifting of electrical carriers across a junction interface formed in the light receiving layer;
(3) Crosstalk between adjacent pixels caused by a leak of the electric field; and
(4) Size of a minimum domain of the ferroelectric liquid crystal; etc.
Especially in a spatial light modulator having a reflecting film divided into pixel units, the resolution is significantly lowered by the diffusion of and the drifting of the electrical carriers. The limit of the resolution determined by the above-mentioned elements determines the pixel density of the spatial light modulator.
In a spatial light modulator including a photoconductive layer acting as a rectifier, a ferroelectric liquid crystal layer, and a metal reflecting film provided in a plane sandwiched therebetween and divided into pixel units, an image having high density pixels is generated at a high response speed when the ohmic connection between the photoconductive layer and the metal reflecting film occurs. However, such a spatial light modulator has problems concerning the method for blocking the reading light, high resolution, prevention of generation of defects specific to the ferroeleotric liquid crystal, uniform thickness required for a satisfactory contrast ratio and an excellent alignment, and resistance against impact.
In order to effectively block input light, a construction including a light blocking film formed of an electric insulating material in a uniform thickness provided between a photoconductive layer and a metal reflecting film is disclosed in, for example, Japanese Laid-Open Patent Publication No. 62-40430. Such a light blocking film blocks both of the input light and the output light. Such a construction is advantageous in being easy to produce, but problems occur in that the photoconductivity and the switching performance are declined by accumulation of electric carriers when the spatial light modulator is driven, due to the insulation between the photoconductive layer and the metal reflecting film.
In a spatial light modulator having no light blocking film on pixel portions (for example, the modulator disclosed in Japanese Laid-Open Patent Publication No. 62-169120), there is no need for an electric insulating layer. However, the photoconductive film is formed of a high polymer containing a coloring matter or carbon because a conductive material is not suitable for the photoconductive film. Generally, the light blocking degree is increased in accordance with an increase in the amount of carbon, but the amount ratio of carbon with respect to the high polymer has an upper limit due to .film forming conditions, and the light blocking degree also has an upper limit. In the case when the light blocking film is formed of a metal, substantially complete light blocking is achieved over a visible light region when the thickness of the film is several hundred nanometers. However, according to the conventional spatial light modulators, the output light blocking film cannot be formed of a photoconductive material.